Introduction
For this project, we designed a 2.5 DOF Nerf Gun pen plotter that uses nerf darts to draw pictures on a white board. This plotter utilizes Nucleo STM32476 microcontroller that interfaces with the TMC4210 serial interface IC and the TMC2208 stepper driver, which we used to control our stepper motors. Commands to for specific tasks were sent from our PC to the MCU through serial/uart. In the following sections, we will discuss some of the features of the of the Nerf Gun Pen Plotter. Code specific details can be found in the code documentation.
Project Demo
The Nerf Gun Pen Plotter has two primary functions: Pen Plotting and Find Red.
Pen Plotter
The pen plotter takes in an hpgl file, reads it, and plots according to its instructions. In this demonstration, we have prepared an image that incorporates both a straight lines and curves to demonstrate its drawing capabilites.
A comparison of the image drawn by the nerf gun and the reference image used is shown below. The reference image is overlayed on top of the drawn image.
Obviously, there are several limitations that come with using a nerf gun. From the demonstration, it can be seen that the nerf darts don't always shoot straight. Additionally, the nerf darts themselves are rather large, so there needs to be a significant space between the darts in order to decrease the probability of darts hitting each other. Another issue is that the nerf magizine is only limited to 18 bullets, which forced us to constantly reload the gun. If we had more time, we would have likely designed a larger magazine so the drawing process could have been much smoother. Lastly, the suction on the darts were not great either, causing some darts to fall off after a while.
Finding Red
An additional functionality that we added was the ability for the nerf gun to automatically detect and shoot at the color red.
To accomplish this functionality, we used a webcam that was attached to the side of the gun. Using a program ran on the computer, we utulized OpenCV modules to help detect the color red. Then using the field of view, direct anglular instructions were sent from the PC to the MCU instead of cartesian coordinates. Additional details can be found in the camera documentation.
Mechanical Design
The following sections will discuss the different mechanical design elements used in our project.
A list of the mechanical components and where it can be bought:
Nerf Gun: https://www.amazon.com/Turbine-Motorised-Blaster-Customising-Capabilities/dp/B0824SGQP1
NEMA 17 Stepper Motors: https://www.amazon.com/dp/B00PNEQI7W?ref=nb_sb_ss_w_as-reorder-t1_ypp_rep_k0_1_13&&crid=3MQN5NFPV80OP&&sprefix=nema+stepper+
Limit Switches: https://www.amazon.com/MXRS-Hinge-Momentary-Button-Switch/dp/B07MW2RPJY/ref=sr_1_3?keywords=limit+switch&qid=1654909607&sr=8-3
Lazy Susan: https://www.mcmaster.com/lazy-susans/square-turntables-7/width~4/
Nerf Gun
The nerf gun used in this project is the Nerf Elite 2.0 Turbine CS-18 Motorised Blaster.
This is an electrical, automatic nerf gun. In order to access the electronics in this project, the entire back portion of the nerf gun was removed, as shown in the image below.
CAD Design
The main structure of the automatic nerf gun was designed in SolidWorks and then 3D-printed with PLA. Details on assembly can be found in the SolidWorks Assembly file in the repository.
The nerf gun fits in between the two supports. The horizontal and vertical sliders are designed so that the center of gravity of the gun can be found. This help with the stepper motor control so that less torque is needed to move the gun. We found that by having the supports hold the gun at the center of gravity, the stepper motors were less likely to skip steps or fail in holding.
The CAD is also designed to interface with two NEMA 17 stepper motors. These are 4-wire, bi-polar stepper motors. One of the NEMA 17 motors fits onto the top of the side supports to interface with the nerf gun of azimuthal direction control. The other NEMA 17 motor connects the bottom plate and the top plate for polar direction control.
Electrical Design
The automatic Nerf gun incorporates two DC motors to fire bullets: one which spins a set of counter rotating wheels, and one which actuates a piston that pushes darts out of the magazine and into the wheels, launching them out of the barrel of the gun. We had originally planned to incorporate a DC motor driver to control them directly, but upon inspecting the internals of the Nerf gun recognized that there was additional control circuitry. Instead, we opted to simply replace the two switches used as triggers with small signal relays.
Additionally, we observed that in automatic fire mode, the accuracy of the Nerf gun was significantly reduced, and the fire rate was potentially too fast to move our assembly in time to fire each dart in the correct location. To facilitate single fire mode, we would need to “hold” the fire trigger just long enough to fire a single dart, and no longer so that multiple darts weren’t fired by accident. To facilitate this, we implemented an IR breakbeam sensor at the end of the Nerf gun’s barrel. When a dart is fired, the sensor pulls a GPIO pin low, and we can detect this on the MCU via an interrupt. This enabled us to fire a single dart at a time, as well as detect when a misfire or jam occurred, as the dart would not have been detected in a specified time period.
Figure 6 shows the electrical design of the Nerf control board. In addition to the two double-pull double-throw small signals relays used to control the gun, headers for connecting to the IR sensor, Nerf gun internal circuitry, and MCU are included. The board additionally provides power to the Nerf gun, stepping down the 12 volts used to power the stepper motors to 5 volts via the DC-DC converter, and to 6 volts via the linear voltage regulator. While the Nerf gun uses 4 D-cell batteries, totaling 6 volts, we found that it was able to operate at 5V, albeit with slightly higher current draw. The DC-DC converter is able to provide 1.2 amps, while the LDO theoretically provide 1.5 amps, but require a substantial heatsink to achieve this power output. Solder jumpers allow us to select between the two voltage sources, as well as a direct voltage input for use with a second 6 volt power supply.
The schematic and PCB were designed in Altium Designer. Figure X shows the PCB layout in 2-dimensions, while Figure 8 shows a 3D render of the board, including the component bodies. We ordered the PCBs from Osh Park and got SMD stencils from Osh Stencils. Our electronic components were sourced from Mouser, as well as the manufacturer of the DC-DC converter, Monolithic Power Systems. We assembled the boards by hand using a hot air rework station and soldering iron.
Calculations
As part of our drawing task, we developed forward and inverse kinematics for the nerf gun based off of its geometry. The two degrees of freedom we had to account for were the x-angle (polar) and the y-angle (azimuthal). In order to plot an image, we needed to establish a set of equations to related x,y coordinates to angular coordinates. To do this we utilized a Newton-Raphson solver function to iteratively find the actuator angles. The reference drawings we used to do our calcalations are provided below.
Below are the equations that we derived and used in our Newton-Raphson.
Details on how the equations are used can be found in our source code documentation.
Task Diagram
In order to run our code, we utilized a cooperative task schedular in the microcontroller. Data is shared between different tasks so that our program can simultaneously move, calculate coordinates, and receive commands. A task diagram this process is shown below.
- Copyright
- License Info
- Date
- June 10, 2022
